[PATCH 4.8? 0/4] Allow the trampoline to use EFI boot services RAM

[Andy Lutomirski <luto@kernel.org>]

As currently configured, my laptop cannot boot any existing kernel
because the real mode trampoline can't be reserved.  The ranges in
which it could live are rejected by the kernel: one is EFI boot
services data and the other is above the EBDA.

Allowing use of RAM between the EBDA and 640k is scary: there are
probably many quirky BIOSes out there, and, as currently structured,
it would be awkward to allow it just on EFI boots because we
currently reserve that range before we figure out whether we're
using EFI.

This series fixes it the other way: it allow the trampoline to live
in boot services memory.  It achieves this by deferring the panic
due to failure to reserve a trampoline until early_initcall time
and then adjusting the EFI boot services quirk to reserve space
for the trampoline if we haven't already found it a home.

I'm hoping this is okay for 4.8 even though it's late: it fixes
a boot failure and it's fairly conservative -- the only significant
changes in behavior should be on systems that currently fail to boot.

I'm not currently proposing it for stable because AFAIK I'm the
only person to have seen this issue.  If it survives in Linus'
tree for a while, though, I might propose it for -stable later
on.

Andy Lutomirski (4):
  x86/boot: Synchronize trampoline_cr4_features and mmu_cr4_features
    directly
  x86/boot: Defer setup_real_mode() to early_initcall time
  x86/boot: Rework reserve_real_mode() to allow multiple tries
  x86/efi: Allocate a trampoline if needed in efi_free_boot_services()

 arch/x86/include/asm/realmode.h | 10 ++++++++-
 arch/x86/kernel/setup.c         | 17 ++++++++-------
 arch/x86/platform/efi/quirks.c  | 21 ++++++++++++++++++
 arch/x86/realmode/init.c        | 47 ++++++++++++++++++++++++++++++-----------
 4 files changed, 74 insertions(+), 21 deletions(-)

-- 
2.7.4